The art of coal treatment to upgrade coal and provide alternative fuels, particularly liquid fuels to replace petroleum-derived liquid fuels, was first studied intensively in Germany in the 1920's. Research in the technology of coal upgrading has continued up to the present time, and was particularly active during the worldwide oil shortages of the 1970's.
Techniques for recovering more-easily utilized fuels from raw coal are generally known as coal liquefaction. Coal liquefaction can employ a wide variety of non-anthracitic substrates, particularly bituminous, sub-bituminous and lignitic coals. Other organic materials, e.g. peat can also be used.
Coal liquefaction processes broadly include both thermal (non-catalytic) and catalytic procedures. In thermal processes, heat is used to liquefy the coal without addition of extraneous catalytic materials. In thermal coal liquefaction processes, however, minerals, especially iron-bearing species, naturally found in the coal substrate may function as catalysts for the process.
Pier et al. have proposed, in U.S. Pat. No. 2,227,672, thermal treatment of carbonaceous materials, specifically middle oil, tars or coal, with supported sulfide-containing catalysts based on a combination of iron, manganese, copper or zinc; plus a strong hydrogenation catalyst selected from molybdenum, tungsten, cobalt, rhenium, vanadium or nickel.
Schuman et al. (U.S. Pat. No. 3,745,108) have disclosed a process for hydrogenating coal, using a liquid medium containing at least 25% by weight of water and temperatures below about 375.degree. C.
The use of supported catalysts for coal liquefaction has been disclosed by Rieve et al. in U.S. Pat. No. 3,619,404. The process is one in which substantially no liquid slurry material is used and is reported to give lower yields of asphaltenes, than when a slurry medium is used.
Hodgson (U.S. Pat. No. 3,532,617) has recited hydroconversion of coal with a combination of catalysts, one impregnated on the coal and the other supported on a refractory oxide.
Schuman et al, in U.S. Pat. No. 3,183,180, have proposed a process for hydrogenation of oils or coal using, for example, cobalt molybdate on alumina catalyst. Ash, char and unconverted coal are recycled to improve the process.
Garg, in U.S. Pat. No. 4,486,293, herein incorporated by reference, has proposed liquefaction of coal in a hydrogen donor solvent, in the presence of hydrogen and a co-catalyst combination of iron and a Group VI or Group VIII non-ferrous metal or compounds of the catalytic metals.
Coal liquefaction processes attempt to bring about cleavage of weak heteroatom to carbon and strong carbon to carbon linkages in the coal structure. In the context of coal liquefaction, heteroatoms include nitrogen, oxygen and sulfur, bonded in any fashion to carbon of coal. The intermediate free radicals, resulting from cleavage of carbon-heteroatom and carbon-carbon bonds, are hydrogenated during liquefaction to prevent polymerization of the thus-produced free radicals to high molecular weight structures.
Although hydrogen performs the necessary function of hydrogenation in coal liquefaction, it has been found that introduction of hydrogen by a hydrogen donor solvent is preferable to use of gaseous hydrogen alone. Hydrogen donor solvents must dissolve the products from coal liquefaction and must be capable of reversible hydrogenation and dehydrogenation. The donor solvent therefore functions as a hydrogen carrier, upon which hydrogen is loaded and introduced into the reaction mixture. Hydrogenated donor solvent then transfers hydrogen to free radicals generated during coal liquefaction and the hydrogen-depleted solvent is separated from the products and is rehydrogenated before recycling to the coal liquefaction reaction.
Both catalytic and non-catalytic coal liquefaction processes can be performed in a variety of reactors, including slurry phase reactors and fluidized bed reactors.